1. Technical Field
Embodiments of the present inventive concept relate to a method of demodulating a modified dual carrier modulation (MDCM) signal, and more particularly, to an MDCM signal demodulation method for decreasing the complexity of MDCM signal demodulation by dividing a MDCM signal into a real part and an imaginary part and reducing the number of symbol vector candidates.
2. Description of the Related Art
Standards for a multiband-orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) physical layer allows MDCM to be used for transmission at a high speed of 640 to 1024 Mbps.
The MDCM results from modifying conventional dual carrier modulation (DCM) to achieve high speed transmission. According to the MDCM, two independent 16-quadrature amplitude modulation (QAM) symbols are converted into two 256-QAM symbols, which are respectively transmitted over two subcarriers farthest from each other within an OFDM symbol.
When the MDCM is used, an MB-OFDM system can obtain diversity gain to overcome frequency selective fading without performance loss due to the increase in a modulation order.
Conventional techniques for demodulating MDCM signals at a receiving terminal include a maximum likelihood (ML) method, a zero-forcing (ZF) method, and a minimum mean square error (MMSE) method.
When the ML method is used, optimal performance can be provided theoretically but complexity is so high that the ML method is inappropriate to be applied to systems in practice. When the ZF method or the MMSE method is used, the complexity of a system can be decreased, but the performance of the system is deteriorated since diversity gain cannot be obtained.
The standards for the MB-OFDM UWB physical layer define that the MDCM is used together with low density parity check (LDPC) channel coding.
Accordingly, a receiver in an MB-OFDM system needs to carry out LDPC channel decoding. When soft decision demodulation is performed to increase reception performance, an MDCM demodulator needs to transmit an accurate log likelihood ratio (LLR) with respect to bits to an LDPC decoder since the LDPC channel decoding is carried out based on the LLR. When the ML method is used based on even soft decision, there is still a problem of system complexity. When the ZF method and the MMSE method are used, the LLR cannot be generated.